Heat treatment furnace

ABSTRACT

A heat treatment furnace comprising an entry chamber, a vacuum heat treatment chamber and a quenching chamber through which a load to be heat treated can be passed sequentially. A first intermediate compartment is disposed between the entry chamber and the heat treatment chamber with those chambers located at respective adjacent sides of the first intermediate compartment. A second intermediate compartment is disposed between the heat treatment chamber and the quenching chamber which chambers are likewise located at respective adjacent sides of the second intermediate compartment. Means are provided for moving a load progressively from the entry chamber to the quenching chamber through the first intermediate compartment, heat treatment chamber and second intermediate compartment. The entry chamber and the quenching chamber are each capable of being sealed off from the respectively adjacent intermediate compartment by means of a respective plate valve which is disposed in the intermediate compartment concerned and movable towards and away from the diving wall between the compartment and the chamber, the plate valve being movable in a direction perpendicular to the dividing wall. The plate valve carries means for conveying the load into or out of the compartment as appropriate.

United States Patent [191 Knowles Aug. 20, 1974 HEAT TREATMENT FURNACE [76] Inventor: Malcolm Frederick William Knowles, 25 Porgeter St., Stourbridge, England [22] Filed: Aug. 23, 1972 [21] Appl. No.: 283,176

[52] US. Cl 266/5 R [51] Int. Cl C2ld 1/74 [58] Field of Search 266/5 R, 4 R, 4 A, 4 B

[56] References Cited UNITED STATES PATENTS 2,039,203 4/1936 Ogilvy-Webb 266/4 R 2,776,134 1/1957 Wingate 266/4 R 2,804,855 9/1957 Bergman 266/5 R 3,381,947 5/1968 Beggs 266/4 R 3,599,946 8/1971 Westeren.... 266/4 A 3,625,499 12/1971 Westeren 266/4 A 3,633,895 1/1972 Genrich 266/4 A 3,684,263 8/1972 Genrich 266/4 A FOREIGN PATENTS OR APPLICATIONS 9,024 l/l9l3 Great Britain 266/4 B 1,040,271 10/1953 France 266/5 R 699,812 12/1940 Germany 266/5 R Primary ExaminerGerald A. Dost Attorney, Agent, or Firm-Hibben, Noyes & Bicknell [5 7 ABSTRACT A heat treatment furnace comprising an entry chamher, a vacuum heat treatment chamber and a quenching chamber through which a load to be heat treated can be passed sequentially. A first intermediate compartment is disposed between the entry chamber and the heat treatment chamber with those chambers located at respective adjacent sides of the first intermediate compartment. A second intermediate compartment is disposed between the heat treatment chamber and the quenching chamber which chambers are likewise located at respective adjacent sides of the second intermediate compartment. Means are provided for moving a load progressively from the entry chamber to the quenching chamber through the first intermediate compartment, heat treatment chamber and second intermediate compartment. The entry chamber and the quenching chamber are each capable of being sealed off from the respectively adjacent intermediate compartment by means of a respective plate valve which is disposed in the intermediate compartment concerned and movable towards and away from the diving wall between the compartment and the chamber, the plate valve being movable in a direction perpendicular-to the dividing wall. The plate valve carries means for conveying the load into or out of the compartment as appropriate.

6 Claims, 5 Drawing Figures PATENTED RUB 2 01974 3.830.479 sum u or a BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to heat treatment furnaces, and more particularly such furnaces which are required to be capable of operating under vacuum.

Known types of vacuum heat treatment furnace can be classified into two broad divisions, namely those intended for batch-wise operation and those intended for semi-continuous operation.

The present invention is primarily concerned with vacuum heat treatment furnaces of the kind intended for semi-continuous operation, but is also applicable to the kind intended for batch-wise operation.

2. Description of the Prior Art In known vacuum heat treatment furnaces of the kind intended for semi-continuous operation a plurality of successive chambers are provided whereby the work to be treated first enters an inlet chamber which is separated from an evacuated treatment chamber by a sliding plate valve whereby the pressure in the inlet chamber may be raised to atmospheric pressure to enable the work to be inserted. Further treatment chambers may be similarly provided and isolated from one another so that the work may be subjected to several stages of heat treatment at different pressures, or in the presence of different gases as may be necessary. Finally, an exit chamber is provided which may also serve as a quenching chamber.

All these chambers are separated by respective sliding plate valves which are required positively to seal off adjacent chambers from one another.

However, such sliding plate valves are very expensive since they have to be designed not only to slide transversely relative to the pair of chambers which they separate, (i.e., in the plane of the plate), but also to expand or move in the longitudinal direction relative to the path of the work (i.e., perpendicular to the plane of the plate), so as to engage sealingly against corresponding flanges whereby high vacuum conditions may be maintained. Additionally, such sliding plate valves may require cooling means and also heat insulation which fur.- ther complicates their design and renders their sealing action more difficult to achieve.

A somewhat similar construction may also be adapted for a vacuum heat treatment furnace of the kind intended for batch-wise operation, in that such a furnace may include a heat treatment chamber and a separate ante-chamber through which the work is introduced and removed so that the vacuum can be maintained in the heat treatment chamber while work is being loaded into or removed from the furnace.

SUMMARY OF THE INVENTION According to the invention a heat treatment furnace includes two chambers which are, at least at times, required to operate at different pressures and/or with different atmospheres relative to each other, said chambers being located at respective adjacent dividing wallforming sides of an intermediate compartment. Respective first and second transfer means are provided for transferring a load in a first direction from one of said chambers to said intermediate compartment and for transferring said load in a second direction from said intermediate compartment to the other chamber, at least one of said chambers being capable of being sealed off from said intermediate compartment by means of a plate valve which is disposed in said intermediate compartment and is movable generally perpendicularly towards and away from the dividing wall of the compartment adjacent to which the chamber concerned is located whereby said plate valve is movable between an operative position in which it sealingly engages said dividing wall of said intermediate compartment and an inoperative position in which it is disposed adjacent to the side wall of said intermediate compartment which is opposed to said dividing wall.

The chamber which can thus be sealed off from the intermediate compartment can thus serve as, for example, an entry chamber which, when sealed off from the intermediate compartment, can be opened to enable a load to be introduced. The entry chamber can then be evacuated or otherwise purged of air and the plate valve withdrawn to the side wall of the intermediate compartment remote from the entry chamber so that the load can then be transferred from the entry chamber to the intermediate compartment by the appropriate transfer means. The load can then be subsequently transferred from the intermediate compartment to the other chamber which need not necessarily be capable of being sealed off from the intermediate compartment since the latteris itself capable of being sealed off from the first chamber.

This arrangement has the advantage that the plate valvedoes not have to slide transversely into position, but is simply moved directly towards and into contact with the surface against which it is intended to form a seal. Thus there is no abrasion of the sealing surfaces due to relative lateral movement therebetween and an adequate seal can be obtained simply by the use of an appropriately sectioned seal strip carried by either the plate valve or the side wall of the intermediate compartment against which the valve closes.

Conveniently, one of said transfer means comprises a cradle or other structure secured to and movable with the plate valve whereby, as the plate valve is moved from its operative position to its inoperative position, a load carried by said cradleor like structure is thereby transferred from the chamber concerned to the intermediate compartment.

The. chamber in which heat treatment of the load is carried out would normally include internal insulation, and preferably a slab of insulating material is provided which is displaceable laterally into and out of an operative position in which it is disposed between said heat treatment chamber and said intermediate compartment in non-sealing relation.

Thus it is only necessary for the slab of insulating material to be mounted in such a manner that it can be slid into a position to close the otherwise open side wall of the intermediate compartment through which the load is introduced into, or removed from the heat treatment chamber so as to prevent heat loss by radiation in that direction. It is not necessary for the slab of insulating material to engage the side wall of the remaining insulating material in sealing relationship.

However, the slab of insulating material is preferably arranged to come into light contact with the remaining insulating material so that, when it is in its operative position, the insulating material defines an innner enclosure within the heat treatment chamber. In this case, a gas inlet may be provided leading to the interior of the inner enclosure so that a selected gas or atmosphere the interior of said inner enclosure may be placed in communication with the outer part of the heat treatment chamber and a pumping system connected thereto so that said gas or atmosphere introduced into said inner enclosure can subsequently be evacuated. Impeller means may be provided within the inner enclosure to circulate the gas or atmosphere over the load therein.

It is to be understood that the inner enclosure need not be completely sealed so as to prevent any escape of the gas to the outer part of the heat treatment chamber.

Thus, for treatment at below atmospheric pressure it is necessary only for the escape of gas from the inner enclosure to be limited to a rate which the pumping system of the chamber can cope with. However, for treatments at or above atmospheric pressure the pumping system would be inoperative, and the intermediate compartment would also contain the gas or atmosphere introduced into the inner enclosure, the plate valve sealing off the adjacent chamber.

It is to be understood that a vacuum heat treatment furnace in accordance with the invention may comprise a plurality of chambers through which a load to be treated is passed in sequence via respective intermediate compartments between such chambers'and each incorporating a respective plate valve which is sealingly engageable with a side wall of the intermediate compartment concerned.

A further feature of the present invention is that a chamber, other than the heat treatment chamber may be adapted to serve as a quenching chamber. In this case means may be provided for supplying a quenching fluid to such chamber when the plate valve associated therewith is closed to seal off that chamber from the intermediate compartment.

Means may be provided for introducing a selected gas or atmosphere into the quenching chamber, and additionally or alternatively means may be provided for introducing a quenching liquid into the quenching chamber. Impeller means may be provided for circulating the quenching fluid within the quenching chamber. Alternatively means may be provided for circulating the quenching fluid through the quenching chamber. Hot quenching liquid could be employed.

According to a further feature of thepresent invention a means may be provided for introducing a selected gas or atmosphere into the heat treatment chamber.

Such a provision enables a load to be heat treated under a selected atmosphere without the necessity of a lengthy purging operation after the load has been introduced into the chamber. Normally, the heat treatment chamber for treatment of workpieces in a selected gas or atmosphere requires to be opened to the externalatmosphere to enable the load to be introduced, and it is necessary for the chamber then to be purged by a gas or atmosphere having a composition corresponding to that under which the treatment is to be carried out. Such purging may require to continue for as much as 24 hours in order to remove undesirable traces of oxygen or other gases absorbed in insulating material or brickwork, and only after such lengthy purging operation can the heat treatment commence.

However, when using the furnace in accordance with the invention, and having means for introducing the selected gas or atmosphere into the heat treatment chamber, the heat treatment chamber can be isolated completely from the entry chamber by the plate valve whilst the load is placed in the entry chamber. It is then merely necessary to evacuate or otherwise purge the entry chamber and transfer the load to the heat treatment chamber. Only a short purging period, typically lessthan an hour, is then required before the heat treatment can commence under the selected gas or atmosphere. Preferably, of course, the heat treatment chamber is evacuated during the purging period.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the accompanying drawings wherein: 7

FIG. 1 shows one embodiment of vacuum heat treatment furnace in perspective view and partly broken away to show the internal construction,

FIG. 2 shows diagrammatically a transverse section through the heat treatment chamber of the furnace shown in FIG. 1, and

FIG. 3 shows a similar diagrammatic section through a heat treatment chamber of an alternative embodiment,

F IG. 4 shows a longitudinal section through a further embodiment, and

FIG. 5 shows a partially sectioned plan view corresponding to FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The furnace illustrated in FIG. 1 includes an entry chamber 10, a heat treatment chamber 11 and an quenching chamber 12 which also serves as an exit chamber. A first intermediate compartment 13 is disposed above the entry chamber 10 and at one end of the heat treatment chamber 11, whilst a second intermediate compartment 14 is disposed at the other end of the heat treatment chamber 11 above the quenching chamber 12.

The entry chamber 10 includes a door 15 and a load to be heat treated can be inserted into the chamber 10 when the latter is at atmospheric pressure. In order to isolate the chamber 10 from the heat treatment chamber 11 so that the latter may be maintained as a vacuum, a plate valve 16 is pressed downwardly by means of a ram 25 into contact with the lower side wall 17 of the compartment 13 which divides that compartment from the entry chamber so as to seal against the dividing wall 17, the plate 16 carrying a sealing strip 18 for this purpose.

The dividing wall '17 is centrally apertured and the plate 16 carries at its underside load supporting means comprising a cradle 19 which projects through the aperture in the wall 17'into the inlet chamber 10. The cradle comprises four corner posts 20 carrying at their lower ends a section of roller track conveyor 21.

It will therefore be understood that when the plate 16 is in its lowered, or operative position, the entry chamber 10 can be raised to atmospheric pressure even though the heat treatment'chamber 11 and the first intermediate compartment 13 are evacuated providing the force applied by the hydraulic ram to the plate 16 is sufficient to overcome atmospheric pressure thereon at the underside.

After a load has been inserted into the chamber onto the roller track section 21, the chamber 10 can be evacuated and then the plate valve 16 can be raised to its inoperative position in which it is disposed adjacent to the upper side wall 22 of the compartment 13. The load on the track section 21 is thus lifted into the compartment 13.

The side wall 23 of the compartment 13 which divides that compartment from the heat treatment chamber 11 is apertured so that the load may be transferred into the heat treatment compartment by means of a pusher device (not illustrated) operated through a hydraulic ram as indicated at 24 mounted at the outer side wall 26 of the compartment 13.

The heat treatment chamber contains heating electrodes 27 and a layer of insulating material 28 is disposed between the electrodes and the outer walls of the chamber which is open at both ends. in order to prevent heat loss by radiation additional slabs of insulating material are provided at each end of the heat treatment chamber. One of such slabs is shown at 29.

Each slab is capable of being withdrawn upwardly into a housing 30 by means of a hydraulic or pneumatic ram or other actuator 31. Thus, when the slab 29 at the end of the heat treatment chamber 11 nearest the intermediate compartment 13 is raised, a load can be transferred from the cradle 19 into the heat treatment chamber and the slab can susequently be lowered. Due to the provision of the plate valve 16 a further load can be introduced into the entry chamber 10 whilst the previous load is being heat treated, and it is not necessary for the heat treatment chamber 11 to be sealed off from the compartment 13.

The arrangement of the second intermediate compartment l4 and the quenching chamber 12 is generally similar to that of the first intermediate compartment l3 and the entry chamber 10. Thus, the compartment 14 contains a plate valve 32 which can be raised and lowered by a ram 33 so as to seal off the quenching chamber 12 by engagement with the apertured wall 34 which divides the compartment from the quenching chamber.

The plate valve 32 carries a cradle 35 similar to the cradle 19 and also incorporating a section of roller conveyor track. Additionally, the compartment 14 is provided with a retriever device (not shown) which is operated by a ram 36 supported on the side wall of the compartment 14 remote from the chamber 11. The retriever device is such that, when the slab 29 at the end of the chamber 11 nearest the compartment 14 is raised, it can be advanced into the chamber 11 to connect with the load therein and thus withdraw the load into the second intermediate compartment 14. It will be understood that this operation is performed with the plate valve 32 in its raised position and the quenching chamber 12 evacuated. In this way the load is transferred from the heat treatment chamber 11 to the cradle 35 in the intermediate compartment 14.

Subsequently, the plate valve can be lowered so that the cradle descends into the quenching chamber 12 and thus seals off the compartment 14 from that chamber. The quenching fluid can then be introduced into the chamber 12.

The chamber 12 includes an inlet (not illustrated) whereby any selected gas or atmosphere may be admitted to the quenching chamber, and an impeller 37 is provided in one side wall of the chamber to cause such gas or atmosphere to circulate over the load. Additionally, the chamber 12 is provided with a pump 37a, an inlet 38 and the outlet 39 whereby a quenching liquid can be circulated through the chamber 12. Thus the load may be quenched by inert gas, non-inert gaseous mixtures, air or liquids, or subjected to quenching operations involving combinations of such media. The quenching liquid can be circulated at temperatures up to 200 250 C.

Whilst the quenching process is in operation, a load which has previously been introduced into the entry chamber 10 can, of course, be transferred into the heat treatment chamber 11 since the latter is at this time sealed off from the quenching chamber 12.

it will therefore be apparent that the furnace is capable of semi-continuous operation but avoids the use of expensive sliding plate valves of the type previously employed to separate adjacent chambers. Since the slabs 29 of insulating material are not required to engage sealingly against the side walls of the intermediate compartment, or against the remaining insulating material 28, such slabs are not subject to any undue wear and do not require complicated mechanisms to press them laterally against a co-operating sealing surface.

However, the slabs 29 are preferably arranged so that, when in their lowered operative position, they engage lightly against the end of the box-like structure afforded by the insulating material 28 so as to define an inner enclosure within the chamber 11. It is to be understood that such inner enclosure is not sealed and not leak-proof.

When the furnace is provided with an inner enclosure in this manner, it can be modified in the manner illustrated diagrammatically in FIG. 3. In this modification a gas inlet 40 is provided leading into the interior of the inner enclosure, and a port 41 is formed in the insulating material in alignment with a duct leading to a vacuum pumping system whereby the heat treatment chamber 11 can be evacuated. A plug 43 is provided to close the port 41, and an impeller 44 is disposed adjacent to the upper wall of the enclosure.

This arrangement enables a selected gas or atmosphere to be introduced into the inner enclosure after 7 it has been evacuated, the plug 43 being open during the evacuation procedure, and closed during the subsequent introduction of gas. Thus the heat treatment can be carried out under any selected atmosphere at pressure up to and above atmospheric pressure.

Whilst the inner enclosure is not leak-proof, it can readily be constructed in such a manner that for treatments at reduced pressure and when the slabs 29 are lowered, the rate of escape of gas from the interior of the enclosure is sufficiently low for it to be handled easily by the pumping system so that a vacuum is maintained in the outer part of the chamber 11 and the adjacent compartments 13 and 14, the pumping system having sufficient capacity in reserve to evacuate the chamber 10 or 12 when necessary. However, where treatments are to be carried out at or above atmospheric pressure, the pumping system is inoperative so that the gas is permitted to accumulate in the intermediate compartments. However, such gas will cool rapidly and there will be little heat transfer by radiation due to the slabs 29 so that no damage will be caused to the plate valves and rams etc. Moreover, the chambers 10 and 12 are isolated by the plate valves. At the end of the heat treatment in chamber 11, the pumping system is used to evacuate the chamber and the intermediate compartments.

Since the chamber 11 can be maintained continuously under vacuum when the load is being transferred thereto it does not become contaminated with gasesfrom the external atmosphere so that no prolonged purging period is necessary before the heat treatment operation can commence.

Referring now to FIGS. 4 and 5, there is shown therein the heat treatment chamber and associatedintermediate compartments of a further embodiment of heat treatment furnace in accordance with the invention. The general construction of this embodiment is closely similar to that shown in FIG. 1 and equivalent parts are identified by the same reference numerals as used previously so that the preceding description is deemed to apply. The following description relating to FIGS. 4 and 5 concerns only features which are shown in FIGS. 4 and 5 and not in FIG. 1.

Referring firstly to the modified cradle 19 within the intermediate compartment 13, it will be observed that the plate valve 16 comprises two plates 16a and 16b in face-to-face contact. Between them, the two plates 16a and 16b define an internal passageway through which a cooling fluid can be circulated. Such fluid can be supplied through a tube 25b which is coaxially arranged within a bore 250 extending longitudinallythrough a rod 25a to which the plate 16 is secured. The rod-25a extends through the ram 25 to enable a supply of cooling fluid to be connected thereto at its upper end. The circulating cooling fluid is returned through the annular space defined between the tube 25b and the bore 25c.

In order to protect the sealing strip 18 from heat radiated from the heat treatment chamber 11, when the plate 16 is raised, shields 18a are provided. Each shield 18a comprises a generally V-section strip having an inclined marginal portion adapted to engage the underside of the plate valve 16 at a position inwardly of the sealing strip 18, as can clearly be seen from FIG. 4. The shields 18a are secured to carrier bars 18b having end flanges 18c through which extends a pivot pin 180'. The carrier bar 18b includes an inwardly directed arm 18e which engages a rib formed on the upper plate 16a of the plate valve 16 and also serves to bias the shield 18a gravitationally to swing downwardly away from the sealing strip 18.

The arrangement is such that when the plate valve 16 is raised, the arm 182 engages the rib so as to lift the shield 180 into its operative position as shown in FIG. 4. When the plate valve 16 is lowered, the arm 18e swings downwardly so that the shield 18a swings downwardly and outwardly so as to clear the periphery of th' plate valve. However the arm 18s is so dimensioned as to be engaged at its extremity when the plate valve 16 is raised and thus swing the shield 18a inwardly beneath the sealing strip 18.

The cradle 19, instead of incorporating a length of roller track 21, may alternatively comprise a base plate 19a supporting thereon two spaced parallel sections of rail track 19b. The track can thus support a wheeled trolley 19c to carry the load to be treated. As can be seen, the base plate 190 conveniently is of somewhat larger dimensions than the aperture in the bottom wall 17 of the intermediate compartment 13 and it would accordingly be possible to provide a sealing strip so as to seal the intermediate chamber 13 from the entry chamber 10 when the plate valve 16 is in its raised position should this be desirable for any reason.

The trolly includes a pair of spaced flanges 19d adapted to receive therebetween a pusher head 24b carried by the rod 240 of the hydraulic ram 24.

To ensure that the trolley 19c is correctly located, so that the pusher head 24b engages between the flanges 19d as the cradle 19 is raised, the trolley 19c is equipped with a block l9e formed with a recess 19f adapted to receive a locking plunger 19g carried by a counterweighted level 19h. The counterweighted lever 19h biases the locking plunger 19g upwardly to engage the block 19c and hold the trolley 19c stationary. However, the locking plunger 19g carries a projecting arm 19j which is arranged to engage the underside of the lower wall 17 of the intermediate compartment 13 as the cradle 19 is raised. The plunger 19g thus withdrawn from the block 19c as the plate valve 16 moves into its fully raised position and the trolley 190 is freed for movement by the pusher head 24b.

An intermediate section of rail track 52 is disposed between the compartment 13 and the chamber 11 and the latter contains a further section 53 of rail track so that the trolley 19e may be transferred into the heat treatment chamber. The rail track sections 52 and 53 are separated by a gap sufficient to receive the insuulating slab 29 when this is lowered as shown in FIG. 4.

The heat treatment chamber 11 includes a work supporting hearth 54 which is capable of being raised and lowered by a pair of hydraulic actuators 55. The hearth 54 is so shaped relative to the trolley 19c as to lift the load'from the latter as the hearth is raised. The trolley can then be withdrawn into the compartment 13.

The assembly of plate valve 32 and cradle 35 in the intermediate compartment 14 is exactly similar to that in the intermediate compartment 13 and therefore will not be described in detail. However, the trollery 35c, which is exactly similar to the trolley 190, is clearly shown in plan view in FIG. 5. As can be seen, the trolley 35c is supported by rails 35b carried by the base plate 35a. The pusher head 36b of the rod 36a of the ram 36 engages between the flanges 35d at one end of the trolley whilst the latter is provided with a longitudinally extending central slot. The trolley 350 is intended to support a load which, as indicated by the lines 50 extends laterally beyond the edges of the trolley. The trolley 350 can thus be advanced into the heat treatment chamber 11 whilst the hearth 54 is in its raised position with the load supported thereon. it will be understood that the hearth incorporates three longitudinally extending ribs of which the inner can be received within the longitudinal slot of the trolley 350. The outer ribs are spaced apart by a distance sufficient to enable the trolley 35 c to be received therebetween. Thus, when the hearth 54 is lowered the load thereon is transferred to the trolley 350 which can then be withdrawn into the intermediate compartment 14 and subsequently lowered into the quenching chamber 12.

The casing of the heat treatment compartment 11 is equipped with a cooling coil 55 and the electrodes (not shown) are supplied through terminals 56. For each of maintenance, the terminals 56, electrodes, hearth 54, actuators 55 and rails 53 are all mounted on a common base plate 57 so that they can readily be withdrawn from the chamber 11 as a single unit.

it will be understood that, if semi-continuous operation were not required, the furnaces illustrated could be modified by eliminating the second intermediate compartment 14 and the quenching chamber 12, and

modifying the entry chamber 10 so that it would also serve as a quenching chamber. Work to be treated would then be returned to the chamber 10 after heat treatment in the chamber 11 and there subjected to the quenching operation.

Where several stages of heat treatment were required, it would, of course, be possible for a plurality of heat treatment chambers to be provided, each being separated from adjacent chambers by additional intermediate compartments equipped with plate valves.

1 claim:

1. A heat treatment furnace for treating a load of material or workpieces comprising,

a. a furnace chamber which is adapted to operate at a subatmospheric pressure and an access chamber which is adapted to be opened to the external atmosphere for transit of said load,

b. heating means for supplying heat to said furnace chamber,

c. an intermediate compartment having a plurality of interconnected sides, said two chambers being respectively disposed at a pair of adjacent said sides of said intermediate compartment, and each of the adjacent said sides being apertured to permit transfer of a said load therethrough,

d. first transfer means for transferring said load between said access chamber and said intermediate compartment, and second transfer means for transferring said load between said intermediate compartment and said furnace chamber,

e. means for sealing off said access chamber from said intermediate compartment adapted to maintain when in an operative position a vacuum in said compartment, said means comprising a plate valve disposed internally of said intermediate compartment, a co-operating seating for said plate valve at the side at which said access chamber is located, and means for holding the plate valve when in said operative position in pressure engagement with said seating sufficient to maintain a controlled pressurein said compartment against a differential pressure in said access chamber and for moving the plate valve generally perpendicularly towards and away from said seating between said operative position and an inoperative position in which the plate valve is spaced from said seating internally of said intermediate compartment and adjacent to the side of the intermediate compartment which is opposed to said side at which the seating is provided, and

f. said furnace having associated therewith means adapted to form a vacuum therein and inlet means for introducing into the furnace chamber a selected gas or atmosphere at a controlled pressure.

2. A furnace according to claim 1, wherein said first transfer means comprises a load supporting means secured to said plate valve in vertically spaced relation therewith to define a load receiving space between said plate valve and said load supporting means, the load supporting means being movable with the plate valve whereby, as the plate valve is moved between said operative and said inoperative positions, said load supporting means and a said load within said load receiving space is transferred between said intermediate compartment and said access chamber.

LII

3. A furnace according to claim 2, wherein the load supporting means comprises a section of a rail track and a trolley movable over said rail track, said second transfer means comprising a further section of rail track which extends from said intermediate compartment into said furnace chamber and, within the latter, a work supporting a hearth which is vertically movable and so shaped relative to the trolley as to enable said load to be transferred between the trolley and said hearth.

4. A furnace according to claim 1, wherein the furnace chamber further comprises a. internal insulation which defines an inner enclosure within said furnace chamber and including a slab of insulating material which is displaceable laterally into and out of an operative position in which it is interposed between said furnace chamber and said intermediate compartment in non-sealing relation,

a closable port in said insulating material whereby the interior of said inner enclosure may be placed in communication with an outer part of the chamber externally of said inner enclosure, and

c. said inlet means'comprising a gas inlet leading to the interior of the inner enclosure so that said selected gas or atmosphere can be introduced therein at a controlled pressure.

5. A furnace according to claim 1, wherein said access chamber is also adapted to serve as a quenching chamber and means are provided for supplying a quenching fluid to said chamber when the plate valve is closed to seal off said access chamber from the intermediate compartment.

6. A heat treatment furnace according to claim 1, further comprising a. a further access chamber which is adapted to be opened to the external atmosphere for transmit of said load, said first access chamber serving to admit said load to the furnace and said further access chamber serving for withdrawal of said load from .the furnace after heat treatment therein,

b. a further intermediate compartment having a plurality of interconnected sides; said furnace chamber and said further access chamber being respectively disposed at a pair of adjacent said sides of said further intermediate compartment, each of the adjacent said sides of said further intermediate compartment being apertured to permit transfer of said load therethrough,

c. third transfor means for transferring said load from said furnace chamber into said further intermediate compartment, and fourth transfer means for transferring said load from said further intermediate compartment into said further access chamber, said first transfer means serving to transfer said load from said first access chamber into said first intermediate compartment, and said second transfer means serving to transfer said load from said first intermediate compartment into said furnace chamber, and

d. means for sealing off said further access chamber from said further intermediate compartment adapted to maintain a vacuum in said further intermediate compartment, said means comprising a second'plate valve disposed internally of said further intermediate compartment, a co-operating second seating .for said second plate valve at the tive position and an inoperative position in which the second plate valve is spaced away from said second seating internally of said further intermediate compartment and adjacent to the side of the further intermediate compartment which is opposed to said side at which the second seating is provided. 

1. A heat treatment furnace for treating a load of material or workpieces comprising, a. a furnace chamber which is adapted to operate at a subatmospheric pressure and an access chamber which is adapted to be opened to the external atmosphere for transit of said load, b. heating means for supplying heat to said furnace chamber, c. an intermediate compartment having a plurality of interconnected sides, said two chambers being respectively disposed at a pair of adjacent said sides of said intermediate compartment, and each of the adjacent said sides being apertured to permit transfer of a said load therethrough, d. first transfer means for transferring said load between said access chamber and said intermediate compartment, and second transfer means for transferring said load between said intermediate compartment and said furnace chamber, e. means for sealing off said access chamber from said intermediate compartment adapted to maintain when in an operative position a vacuum in said compartment, said means comprising a plate valve disposed internally of said intermediate compartment, a co-operating seating for said plate valve at the side at which said access chamber is located, and means for holding the plate valve when in said operative position in pressure engagement with said seating sufficient to maintain a controlled pressure in said compartment against a differential pressure in said access chamber and for moving the plate valve generally perpendicularly towards and away from said seating between said operative position and an inoperative position in which the plate valve is spaced from said seating internally of said intermediate compartment and adjacent to the side of the intermediate compartment which is opposed to said side at which the seating is provided, and f. said furnace having associated therewith means adapted to form a vacuum therein and inlet means for introducing into the furnace chamber a selected gas or atmosphere at a controlled pressure.
 2. A furnace according to claim 1, wherein said first transfer means comprises a load supporting means secured to said plate valve in vertically spaced relation therewith to define a load receiving space between said plate valve and said load supporting means, the load supporting means being movable with the plate valve whereby, as the plate valve is moved between said operative and said inoperative positions, said load supporting means and a said load within said load receiving space is transferred between said intermediate compartment and said access chamber.
 3. A furnace according to claim 2, wherein the load supporting means comprises a section of a rail track and a trolley movable over said rail track, said second transfer means comprising a further section of rail track which extends from said intermediate compartment into said furnace chamber and, within the latter, a work supporting a hearth which is vertically movable and so shaped relative to the trolley as to enable said load to be transferred between the trolley and said hearth.
 4. A furnace according to claim 1, wherein the furnace chamber further comprises a. internal insulation which defines an inner enclosure within said furnace chamber and including a slab of insulating material which is displaceable laterally into and out of an operative position in which it is interposed between said furnace chamber and said intermediate compartment in non-sealing relation, b. a closable port in said insulating material whereby the interior of said inner enclosure may be placed in communication with an outer part of the chamber externally of said inner enclosure, and c. said inlet means comprising a gas inlet leading to the interior of the inner enclosure so that said selected gas or atmosphere can be introduced therein at a controlled pressure.
 5. A furnace according to claim 1, wherein said access chamber is also adapted to serve as a quenching chamber and means are provided for supplying a quenching fluid to said chamber when the plate valve is closed to seal off said access chamber from the intermediate compartment.
 6. A heat treatment furnace according to claim 1, further comprising a. a further access chamber which is adapted to be opened to the external atmosphere for transmit of said load, said first access chamber serving to admit said load to the furnace and said further access chamber serving for withdrawal of said load from the furnace after heat treatment therein, b. a further intermediate compartment having a plurality of interconnected sides, said furnace chamber and said further access chamber being respectively disposed at a pair of adjacent said sides of said further intermediate compartment, each of the adjacent said sides of said further intermediate compartment being apertured to permit transfer of said load therethrough, c. third transfor means for transferring said load from said furnace chamber into said further intermediate compartment, and fourth transfer means for transferring said load from said further intermediate compartment into said further access chamber, said first transfer means serving to transfer said load from said first access chamber into said first intermediate compartment, and said second transfer means serving to transfer said load from said first intermediate compartment into said furnace chamber, and d. means for sealing off said further access chamber from said further intermediate compartment adapted to maintain a vacuum in said further intermediate compartment, said means comprising a second plate valve disposed internally of said further intermediate compartment, a co-operating second seating for said second plate valve at the side at which said further access chamber is located, and means for holding the said second plate valve in an operative position in pressure engagement with said second seating sufficient to maintain a vacuum in said further intermediate compartment against atmospheric pressure In said further access chamber and for moving the second plate valve generally perpendicularly towards and away from said second seating between its operative position and an inoperative position in which the second plate valve is spaced away from said second seating internally of said further intermediate compartment and adjacent to the side of the further intermediate compartment which is opposed to said side at which the second seating is provided. 